1. Field of the Invention
The present invention relates to standardized mechanical interface systems for reducing particle contamination and more particularly to apparatus employing sealed containers suitable for use in semiconductor processing equipment to prevent particle contamination. Still more particularly, the invention relates to such systems enabling direct transfer, without use of an articulated arm, of multiple semiconductor wafers at a time between a transportable container or carrier and a controlled environment load lock chamber as they await further transfer to a processing station, or elsewhere. Throughput of the manufacturing process is thereby significantly increased while simplifying the mechanisms involved.
Throughout this disclosure, the term "wafer" will be used for purposes of consistency to refer to planar substrates such as silicon wafers and glass flat panels, but it will be understood that it is intended to be used in the broad context so as to be applicable to all substrates. Such substrates are circular and were previously sized to a diameter of 200 mm and a thickness of approximately 0.760 mm although, currently, the diameter of choice has evolved to 300 mm with the same thickness.
2. Description of the Prior Art
Control of particulate contamination is imperative for cost effective, high-yielding and profitable manufacturing of VLSI circuits. Because design rules increasingly call for smaller and smaller lines and spaces, it is necessary to exert greater and greater control on the number of particles and to remove particles with smaller and smaller diameters.
Some contamination particles cause incomplete etching in spaces between lines, thus leading to an unwanted electrical bridge. In addition to such physical defects, other contamination particles may cause electrical failure due to induced ionization or trapping centers in gate dielectrics or junctions.
The main sources of particulate contamination are personnel, equipment, and chemicals. Particles given off by personnel are transmitted through the environment and through physical contact or migration onto the wafer surface. People, by shedding of skin flakes, for example, are a significant source of particles that are easily ionized and cause defects.
Modern processing equipment must be concerned with particle sizes which range from below 0.01 micrometers to above 200 micrometers. Particles with these sizes can be very damaging in semiconductor processing. Typical semiconductor processes today employ geometries which are 1 micrometer and under. Unwanted contamination particles which have geometries measuring greater than 0.1 micrometer substantially interfere with 1 micrometer geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries.
In the recent past, "clean rooms" were established in which through filtering and other techniques, attempts were made to remove particles having geometries of 0.03 micrometer and above. There is a need, however, to improve the processing environment. The conventional "clean room" cannot be maintained as particle free as desired. It is virtually impossible to maintain conventional clean rooms free of particles of a 0.01 micrometer size and below. Although clean room garments reduce particle emissions, they do not fully contain the emissions. It has been found that as many as 6000 particles per minute are emitted into an adjacent one cubic foot of space by a fully suited operator.
To control contamination particles, the trend in the industry is to build more elaborate (and expensive) clean rooms with HEPA and ULPA recirculating air systems. Filter efficiencies of 99.999% and up to ten complete air exchanges per minute are required to obtain an acceptable level of cleanliness.
Particles within the equipment and chemicals are termed "process defects."
To minimize process defects, processing equipment manufacturers must prevent machine generated particles from reaching the wafers, and suppliers of gases and liquid chemicals must deliver cleaner products. Most important, a system must be designed that will effectively isolate wafers from particles during storage, transport and transfer into processing equipment. The Standard Mechanical Interface (SMIF) system has been devised, and used, to reduce particle contamination by significantly reducing particle fluxes onto wafers. This end is accomplished by mechanically ensuring that during transport, storage and processing of the wafers, the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient outside environment do not enter the immediate internal wafer environment.
The SMIF concept is based on the realization that a small volume of still, particle-free air, with no internal source of particles, is the cleanest possible environment for wafers.
A typical SMIF system utilizes (1) minimum volume, dustproof boxes or carriers for storing and transporting (2) open rack wafer cassettes, and (3) doors on the boxes or carriers designed to mate with doors on the interface ports on the processing equipment and the two doors being opened simultaneously so that particles which may have been on the external door surfaces are trapped ("sandwiched") between the doors.
In a typical SMIF system, a box or carrier is placed at the interface port and latches release the box door and the port door simultaneously. A mechanical elevator lowers the two doors, with the cassette riding on top. A manipulator picks up the cassette and places it onto the cassette port/elevator of the equipment. After processing, the reverse operation takes place.
SMIF systems have proved to be effective and this fact has been shown by experiments using SMIF components both inside and outside a clean room. The SMIF configuration achieved a ten-fold improvement over the conventional handling of open cassettes inside the clean room.
Using SMIF systems, it was customary to carry a large number of the wafers within the box or carrier by supporting them in a spaced relationship by means of a cassette. Using this technique, the cassette would be loaded with a supply of wafers, transported into the box or carrier, then subsequently wafers would be removed from the cassette in the carrier one by one for placement into a reception chamber at the site of further processing.
A recent development relating to SMIF systems is presented in U.S. Pat. No. 5,752,796 to Muka which discloses the use of such a portable carrier for transporting multiple semiconductor wafers in a substantially particle free environment. A cassette freely received within the carrier for supporting the wafers in spaced generally stacked relationship is delivered to an environmentally clean load lock. A mini-environment defines an interior region adjacent the load lock for the engageable reception of the carrier and sealingly isolates the load lock chamber and the interior of the carrier from the surrounding atmosphere. Transfer mechanisms serve to retrieve the cassette from the carrier and move the carrier into the load lock chamber while maintaining it in a particle free environment. A first transport mechanism within the mini-environment retrieves the cassette from the carrier and moves it into its interior region and a second transport mechanism within the load lock retrieves the cassette from the interior region of the mini-environment and moves it into the load lock chamber. To aid in moving the cassette into the interior region, a hood of the mini-environment is movable between lowered and raised positions while maintaining a capillary seal with its upstanding walls. Sensors inform whether a cassette is present in the SMIF box, whether wafers are properly positioned in the cassette, and whether the cassette is properly gripped as it is being moved into the interior region. Laminar flow air is continually directed through the interior region of the mini-environment and filtered to remove foreign particles from the wafers being supported in the cassette.
The invention results in an efficient, rapid operating, unified and simplified design which assures maximum throughput of processed wafers while protecting them from the outside environment throughout performance of the process steps. It moves a cassette containing a plurality of wafers into the vacuum lock, while vented, while maintaining the clean conditions of the SMIF box by utilizing the load arm already available in existing load lock constructions instead of a separate arm in a SMIF unloader or within the mini-environment. Additionally, the system of the invention introduces progressive cross flows of air as the SMIF box is opened and the cassette with its multiple wafers therein is brought into the mini-environment.
Unfortunately, the use of cassettes requires additional equipment inventory, increases the volume requirement for the box or carrier, adds to the complexity of the system, increases the cost of original equipment and for upkeep of the equipment of the system. Also, risks of contamination, notwithstanding vigorous air circulation, are always present when using cassettes because the plastic material of which they are constructed often outgasses and it is difficult to control such an occurrence.
More recent improvements have eliminated the need for cassettes and examples of such newer technology is found in a number of commonly assigned U.S. Patents, for example, U.S. Pat. No. 5,609,459 to Muka, and U.S. Pat. Nos. 5,607,276, 5,613,821, and 5,664,925 to Muka et al.
It was in light of the foregoing state of the art that the present invention has been conceived and is now reduced to practice. Specifically, the invention results from efforts to reduce equipment inventory and therefore initial cost, provide a simpler and more compact construction, reduce the cost of maintenance, and increase throughput of processed items.